1. Field of the Invention
The present invention relates to a polar leapfrog filter which can be constructed in the form of an active filter, and more particularly it pertains to an even-order polar leapfrog low-pass filter.
2. Description of the Prior Art
Heretofore, such a passive filter as shown in FIG. 1 has been widely used. However, it is the recent trend that so-called active filters are employed in lieu of such passive filters, as the result of peripheral circuits associated therewith being constructed in the form of semiconductor integrated circuit. Generally, an active filter is made up of components, each of which comprises a resistor, a capacitor, and an operational amplifier, and constructed in the form of a sallen-key circuit, a biquad circuit or an FDNR (frequency-dependent negative resistance) circuit by combining such operational amplifiers. Alternatively, a desired filter is constructed by using such circuits as units. In an attempt to change the filter characteristics, with the biquad circuit or the like, it is required that the constants for the resistors and capacitors be changed. With the FDNR filter, on the other hand, difficulties are experienced in an attempt to adjust the filter characteristics thereof since it is the usual practice that several such filters are interconnected with each other and it is required that constants for the elements of each such filter be changed. The other types of filter use variable resistors to make variable the filter characteristics thereof; thus, such filters are constructed inevitably in the form of a hybrid integrated circuit. Alternatively, it is required that chip components of a predetermined resistance value be pre-selected and mounted onto a printed circuit board, which disadvantageously leads to a increase in the size of the filter. In either case, such filters are disadvantageous in that they cannot be constructed in the form of a monolithic integrated circuit since the variable resistors or the pre-selected chip components should be mounted onto the printed circuit board as mentioned above.
In an attempt to make a Cauer filter or the like or achieve desired filter characteristics, it is required that the filter be constructed in the form of polar type having a damping pole, i.e., transmission zero point (pole zero) at a definite frequency. With the above-mentioned conventional arrangements, however, when it is attempted to achieve this, problems arise in that a number of parts are required so that the circuit arrangement turns out to be complicated and difficulties are encountered in an attempt to achieve an an odd-order arrangement, though an even-order arrangement is readily achievable, as is the case with a biquad circuit. The problems with a filter constructed in the form of a hybrid integrated circuit having a number of parts mounted on a printed circuit board, are such that most such filters are bulky and difficult to adjust the filter characteristics thereof.